make objects appear in a flash
Using Cycles materials

Create the appearing object with material that is a combination of emitter and diffuse, and mix that with a transparent material.
At the beginning of the animation the material should be completely transparent:

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Animate the values for the emitter and the transparency so that as it becomes more visible it also becomes brighter with the emitter. Once it reaches the brightest value, start taking the emitter back to zero so that it becomes diffuse only with no transparency.

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For added visual punch swich over to the compositor, where you can add a glare effect (ShiftA &gt; Filter &gt; Glare) to the whole thing.


Using the compositor only
Separate the objects to appear in a different render layer.

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Set the alpha transparency so that the layer is invisible at the beginning of the animation.

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Then animate the values for the alpha channel so that the transparency is off and the layer is fully visible but very bright and blured.

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Finally animate the values for the brightness and blur so that the render layer returns to its normal brightness and the blur disappears.